Go to Configuration > Preferences... > Display > Themes
Click the 'Install new' button and locate the theme directory
Make sure 'Enable alpha channel', 'Force scaling' and 'Automatic' are checked. If 'Automatic' scaling results in icons too big or too small for you, uncheck it and try changing the 'Pixels per inch (PPI)' value manually.
The theme can also be installed by root by untarring the package directly into $PREFIX/share/claws-mail/themes/.
It will then be available to every user.
On Windows, "$PREFIX" is the installation directory (usually "C:\Program Files (x86)\Claws Mail").
0.6.4 (2018-11-25): Changed a bit the printer layout.
0.6.3 (2018-11-12): Darkened a bit all the red arrows, now we are there ;-)
0.6.2 (2018-11-11): Lightened all the red arrows.
0.6.1 (2018-11-11): Fixed queue_close_mark.svg that still had the old, a bit smaller clock.
0.6 (2018-11-11): Removed license: this work is to be considered public domain; all arrows are now bi-color; changed "mark [all] unread" colors from red-ish to blue-ish; added a "shine" effect to the eye on "watch/ignore" icons; rounded up a bit the printer icon; changed the inbox colors of tray icon from orange-ish to lightbrown-ish; scaled up a bit the clock on "send queued" icon; lightened some colors; other minor changes.
0.5 (2018-11-07): Changed "reply to all" icon to only show two "faces"; changed yellows to be uniform across all icons; scaled up all folder and inbox icons; changed inbox icons to resemble a bit more an inbox; other minor changes to colors.
0.4.1 (2018-11-03): Some icons still used dark red, changed them too to lighter red.
0.4 (2018-11-03): Lightened reds and greens, changed mark sign to a slanted and curved one, lowered the contrast of white-gray envelopes, the pen is now always slanted right, added CHANGELOG file. Thanks to George Anchev and Michal Suchánek for the suggestions.
0.3 (2018-10-29): Added LICENSE and AUTHORS files.